1. Statement of the Technical Field
The inventive arrangements relate to detection of radio receiver devices, and more particularly to systems which can use active techniques to determine a range to a radio receiver device.
2. Description of the Related Art
There are many circumstances in which it can be useful to identify the presence of radio receiving devices. For example, identifying the presence of a radio receiver can be useful for many military, law enforcement and homeland security applications. Traditional techniques for performing this function have usually relied on various means for detecting RF emissions from the receiving device. Most often, the source of the RF energy can be directly or indirectly traced to a portion of a radio receiver which is known as a local oscillator.
Local oscillators are commonly included in superheterodyne type radio receivers as part of a down-conversion system. In a superheterodyne radio system frequencies of interest are converted to a constant lower frequency before detection. This constant frequency is called the intermediate frequency, or IF. In this regard, it is well known that certain IF frequencies are commonly selected by designers for certain types of radio receivers.
In order to perform the frequency conversion process from received RF signals to IF, superheterodyne receivers commonly perform a mixing operation that involves two signals. These signals include a signal produced by the local oscillator and an incoming RF signal. This process is performed in a mixer or mixing stage of the receiver. Within the mixer, the local oscillator signal interacts with the received RF signal to produce outputs known as mixing products at frequencies which are equal to the sum of the two input frequencies and difference of the two input frequencies. Other mixing products are also produced that are generally lower in amplitude than the sum and difference products. The receiver includes filtering circuits following the mixer stage to select either the higher or the lower of these frequencies as the IF. Receivers can use one or more mixing stages to produce a desired IF. The IF is typically amplified and is ultimately demodulated by other circuits in the receiver.
Given some information regarding the type of radio receiver to be detected, it is possible to make some reasoned estimates of the likely frequency of the IF signals in a receiver and the local oscillator frequency or frequencies necessary to produce those IF signals. Significantly, signals from the local oscillator and the IF stage of the receiver often radiate from the receiving device. Accordingly, conventional systems have typically confirmed the presence of a radio receiver within an area by using a narrow-band receiver to scan certain frequencies for the presence of such IF signals and/or local oscillator signals. If these emissions are present, it can be presumed that a radio receiver of a particular type is nearby. RF directional finding equipment has also been used in some instances to locate a direction in which such a receiver is located.
Despite the advantages offered by the foregoing systems, they have several important limitations. For example, existing systems do not provide any means for determining a range or distance to a radio receiver. Existing systems lack the ability to resolve directional ambiguities caused by reflections of signals from environmental and man-made structures such as mountains, bodies of water, buildings, airplanes, etc. Existing systems also lack the ability to determine if a target receiver is actually receiving a transmitted signal from a source. Finally, existing systems also do not generally provide any means for determining a minimum required power necessary for transmitted signals to be received by the target receiver.